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Russian forces that seized Ukraine's Zaporizhzhia nuclear power plant have placed staff running the complex under their command and restricted outside communications, the UN nuclear watchdog said on Sunday.
The International Atomic Energy Agency said it was "extremely concerned" about developments at Zaporizhzhia, Europe's largest nuclear power plant, after information from Ukraine's nuclear regulator.
"Ukraine reports that any action of plant management — including measures related to the technical operation of the six reactor units — requires prior approval by the Russian commander," the IAEA said.
"In a second serious development, Ukraine has reported that the Russian forces at the site have switched off some mobile networks and the internet so that reliable information from the site cannot be obtained through the normal channels of communication."
Ukrainian authorities said Russian forces had seized control of Zaporizhzhia on Friday after setting an adjacent training building on fire.
Russia's Defence Ministry blamed the attack on Ukrainian saboteurs, calling it a "monstrous provocation".
The fire was quickly extinguished and there was no damage to reactors or release of radioactive material.
But the incident raised concerns about potentially catastrophic consequences should the conflict damage one of the country's four operating nuclear power plants.
IAEA chief Rafael Grossi voiced his worries over the information received from Ukrainian officials about Russian troops placing staff under their command.
"In order to be able to operate the plant safely and securely, management and staff must be allowed to carry out their vital duties in stable conditions without undue external interference or pressure," Mr Grossi said.
The IAEA also expressed concern about developments at another Ukrainian site seized by Russia, the spent-fuel and radioactive waste centre at Chernobyl, next to the now defunct power plant where the world's worst nuclear accident occurred in 1986.
More than 200 people there, technical staff and guards, have not left since February 23, the day before it was seized, the IAEA said, despite the UN agency's calls for the technical staff to be rotated out on safety grounds.
The Ukrainian regulator said it was "facing problems communicating with personnel" at Chernobyl, the IAEA said. It said communication was only possible through emails.